This invention relates to signal processing and more particularly to a method and apparatus for enabling the transition of an audio data signal converter between the active mode and the inactive mode, based on certain characteristics of the audio data signal. This invention finds applications in digital communication systems, such as a digital cellular system or a Voice-over-IP (VoIP) system, in particular vocoder bypass capable systems that can selectively enable the activation or is de-activation of the decoding and encoding functions in the connection.
In a digital communication system such as a wireless system or a VoIP system, an audio signal may be processed by a series of speech encoders and decoders as it is transmitted from one endpoint to another. In the example of a digital cellular mobile-to-mobile connection, the audio data signal is first encoded by a speech encoder at the first mobile telephone and transmitted in an encoded format to a base transceiver station of a cell site where it is transferred to the base station controller servicing that cell site. At the base station controller, the encoded speech information is processed by a compatible speech decoder that converts the compressed speech stream into PCM samples. The PCM samples are then transported over the landline network, such as the PSTN, toward the base station controller servicing the cell site communicating with the other mobile telephone. At the second base station controller, the PCM speech samples are again processed by a speech encoder. The encoded information is sent from the base transceiver station of the cell site to the second mobile telephone where the compressed speech stream is converted one more time by a speech decoder into PCM samples that can be used to generate an audio signal
In this, codecs on both sides of the mobile-to-mobile call are connected in tandem, which is known to degrade the speech quality as a result of the successive encoding/decoding of the audio data signal.
The xe2x80x9cvocoder bypassxe2x80x9d technique alleviates this problem, specifically when the codecs on both sides of the connection are identical. During a connection, when the codecs at the base station controllers are made aware of their mutual existence, they are switched off such that the encoded speech information arriving at the first base station controller flows in encoded format through the PSTN and arrives as such at the second base station controller. This procedure eliminates one decoding operation of the speech signal at the first base station controller and one re-encoding operation of the signal at the second base station controller. As a result, the audio quality is significantly improved.
When in vocoder bypass mode, the two base station controllers exchange units of compressed data. Each of these units contains an identifier, where this identifier is representative of the compressed state of the data. For each data unit received by the second base station controller when in bypass mode, the identifier is read from the data unit and used to confirm that the unit actually contains compressed information. The data unit is then processed accordingly and transmitted to the second mobile telephone without first being re-encoded, as would occur in non-bypass mode. In the absence of such an identifier, the second base station controller will conclude that the first base station controller is no longer sending compressed data and that communication in the direction from the first base station controller to the second base station controller is in the form of PCM speech samples. Consequently, the second base station controller will switch back to non-bypass mode in that direction.
For additional information on the xe2x80x9cvocoder bypassxe2x80x9d technique, the reader is invited to refer to the U.S. Pat. No 5,768,308 granted to the present assignee that describes the process in great detail. The contents of this document are hereby incorporated by reference.
The codec in one base station controller can switch to the bypass mode as a result of an in-band hand-shaking operation with the codec in the other base station controller. Transmitting control information from one codec to the other over the audio data stream allows this hand-shaking operation to take place. The control information is transmitted by bit stealing. This is effected by inserting in selected PCM samples bits from the control information signal.
Once the handshaking operation is completed, the decoder of the codec in one base station controller and the encoder of the codec in the other base station controller are caused to transition to the inactive mode. This transition may be audibly detectable, in that it may cause distortion over the transmission medium of the connection for a short period of time. This is undesirable as ideally the transition should be made in a manner as transparent as possible to the user.
The present invention provides a signal processor for effecting the conversion of an audio data signal from one format to another. The signal processor has a signal converter that can selectively acquire two operative modes, namely a first operative mode and a second operative mode. In the first operative mode, the signal converter transforms the audio data signal from one format to another and releases the converted audio data signal from the output of the signal processor. In the second operative mode, the signal converter is disabled and permits passage of the audio data signal to the output without conversion.
The signal processor has a control unit for controlling the transition of the signal converter between operative modes. The control unit is responsive to a first control signal representative of a certain characteristic of the audio data signal to enable the signal converter to switch from the active/inactive mode to the inactive/active mode.
The signal processor can find applications in digital communication systems, such as a digital cellular system or a Voice-over-IP (VOIP) system, in particular codec bypass capable systems that can selectively enable the activation or de-activation of the encoding and decoding functions in the connection. In a preferred embodiment, the audio data signal is an encoded signal that includes a succession of data frames. The signal converter has a codec with a decoder, located at a base station of the network that receives the audio data signal from the mobile telephone. In the active mode of the signal converter, the decoder converts the audio data signal into PCM format and sends it to a remote base station over a landline network, such as the PSTN (Public Switched Telephone Network). In the inactive mode, the signal converter passes the encoded audio data, namely the compressed data frames, to the output of the signal processor without decoding the data.
At the remote base station that receives the audio data signal from the first base station, the signal converter has a codec with an encoder. In the active mode of the signal converter, the encoder converts the audio data signal from PCM format to compressed format and sends the encoded data to the corresponding mobile telephone. In the inactive mode, the signal converter passes the encoded audio data received from the first base station to the corresponding mobile telephone without re-encoding the data.
In a specific example, the control signal representative of a certain characteristic of the audio data signal that enables the signal converter to transition from the first operative mode to the second operative mode reflects the type of speech activity in the audio data signal. When the type of speech activity is representative of a certain condition whereby the transition will not harm the audio data signal, such as the absence of speech activity or a low level of speech activity, the control unit allows the transition. This feature is advantageous because the transition is completed in a manner substantially transparent to the user.
The transition from the active mode to the inactive mode may require additional procedures, such as handshaking operations between the signal processors in each base station of the connection. The control signal indicative of the type of speech activity in the audio data signal does not by itself, in such embodiments, effect the transition. The control signal allows the transition to be effected at the opportune time to enhance speech quality.
The invention also provides a method for processing an audio data signal. According to the method, the audio data signal is received and a first control signal representative of a type of speech activity in the audio data signal is provided. By default, the audio data signal is converted from a first format to a second format, where in the first format the audio data signal is compressed data and in the second format the audio data signal is de-compressed data. Conversion of the audio data signal from a first format to a second format can be omitted when the type of speech activity in the audio data signal is representative of a certain condition, such as the absence of speech activity or a low level of speech activity.
The invention also extends to a transmission system using the signal processor described above.
In another example of implementation, the selected characteristic that controls the transition between the operative modes of the signal converter is the format of the audio data signal. Specifically, the audio data signal can be sent under two different conditions. In the first condition, the speech sound information is conveyed under both the first format and the second format. One possibility of accomplishing this is to superimpose the audio data signal in the first format onto the audio data signal in the second format. In the second condition, the speech sound information is conveyed under the second format. The control unit of the signal processor receives the audio data signal from a remote signal processor in either the first condition or the second condition. The control unit determines whether the received audio data signal is in the first or the second condition. If the first condition is detected, the signal converter is set to allow the audio data signal to pass to the output without conversion. If the second condition is detected, the signal converter is set to encode the received audio data signal and pass the compressed data to the output.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.